I never book in peak season, but I would think booking well in advance for summer would be a good idea. I would research more than one itinerary if I were you:
- a round trip YYZ-NRT-YYZ + NRT-ICN-NRT (or any Tokyo airport)
- a multiple YYZ-ICN-NRT-YYZ
I use
http://matrix.itasoftware.com/ with the calendar option